Telephone muffler



Oct. 23, 1928. 1,688,995

r w. F. SMITH. JR

TELEPHONE MUFFLER Filed NOV. 21. 1925 Patented Oct. 23, 1928.

UNITED STATES i 1,688,995 PATENT OFFICE.

WALTER 1'. SMITH, JR, OF NEW YORK, N. Y., ASSIGNOR '10 WESTERN ELECTRIC COH- PANY, INCORPORATED, NEW YORK, N. Y., A CORPORATION OF NEW YOBK.

TELEPHONE MUFFLER.

Application filed November 21, 1925. Serial in. 70,461.

This invention relates to telephone transmitters and more particularly to voice mufflers which enable one to speak into thetransmitter without being overheard by bystanders. These mufilers necessarily entirely enclose the mouth.

An object of the invention is to increase the degree of secrecy obtainable by the use of voice mufliers without sacrificing the eflil0 ciency or the clearness of the telephone conversation. Another object is to improve the quality of the speech transmitted to a telephone line through a voice mufiler.

As is well known, the human voice contains sound vibrations or frequencies ranging from about 50 cycles per second to frequencies asjhigh as 15,000 cycles per second. Most of the energy is contained in sounds below 300 cycles per second. Intelligibility :0 depends on the sounds of the higher frequencies, and therefore sounds of low frequencies, even though radiated to the atmosphere, are unintelligible, but'those of higher frequencies must be suppressed.

In carrying out this invention, a system of filter devices is provided whereby the transmission to the atmosphere of sound waves of high frequencies which would be intelligible is suppressed, and whereby sound. waves of low or unintelligible frequencies, including the hollow sounds caused by thenatural frequency of the mouthpiece, are prevented from exerting their full force upon the diaphragm of the transmitter. In this acoustic filter system no back pressure is generated by the speakers breath in the muflier mouthpiece, as it is thoroughly exhausted in the atmosphere through the lowpass filters.

Inasmuch as the chamber of the mouthpiece which encloses the mouth is resonant at low frequencies, determined by the design of the mouth iece chamber, thus making the transmitte speech unnaturally hols low, a high-pass acoustic filter is preferably introduced between the mouthpiece chamber and the transmitter. This filter effectively attenuates all frequencies below a predetermined value.

by reference to the accompanying drawings in which Fig. 1 is a front assembly view of a mufller embodying certain features of this invention partially shown in section; Fig.

2 is a top view thereof also partially shown This invention will be better understood in section; Fig. 3 is a top assembly view of a modified form of the acoustic filter system shown in Fig. 1 mounted on an ordinary telephone desk set; Fig. 4 is a sectional view taken approximately on line 4-4 of Fig. 3 and looking in the direction indicated by the arrows.

- Similar reference characters refer to similar parts throughout the description.

The telephone 'mufiler of this invention preferably consists of an oval shaped mouth piece 10, on the peripheral edge of which is mounted a band 11 which may be made of any suitable material, preferably felt or rubber, so shaped that it will snugly fit the mouth contour of the operator. The mouthpiece 10 is provided with a wall 12 having a centrally disposed opening 13, in which a bushing 14 provided with a screw threaded nut 15 and collar 15' is adapted to hold the mouthpiece 10 in adjusted position with respect to the particular type of telephone set to which it may be attached.

The opposite portion of bushing 14 is screw threaded and is adapted to fit the threaded portion of the telephone transmitter face late in replacement of the ordinary telep one transmitter mouthpiece. Preferably tangentially disposed on the" mouthpiece 10 and in coaxial alignment are mounted two pairs of identical acoustic filters 16, 17 and 18, 19 shownin Fig. 1. These filters are constructed according to' v the theory and formulae given in a paper by G. W. Steward, Vol. XX, No. 6, of the Physical Review of 1922. Each filter consists of an open tube 20 in which a number of radially disposed apertures such as 21 connect with an associated closed drum 22. In such acoustic filters the dimensions of the tube 20 and the chamber 22 are made such that sound waves contained in the low ran es of frequencies approximately of the or er of up to 300 cycles per second are exhausted through to the atmos here and sound waves 10o contained in the hig er ranges of frequen cies, that is above approximately 300 cycles per second which may be intelligible to persons in the vicinity of the user if permitted I to escape, are strongly attenuated in their 105 passage to the air. It is obvious that a single filter unit of the type described may be made of such. dimensions that it will equal the exhausting capacity of the filters 16, 17, 18 and 19 However, best results have been obtained by using a combination or a system of filters as shown in Fig. 1. By means of this arrangement the unnaturalness of the sound present in mufflers of the type heretofore used due to the back pressure exerted on the transmitter diaphragm is largely eliminated.

Fig. 3 shows a system of filters similar to that shown in Fig. 1 with the addition of a conduit 23 which represents a high-pass filter and which passes sound waves having frequencies above approximately 300 cycles. This tube is coiled and encased in a drum 26 and serves to connect the mouthpiece 10 to the face plate of the telephone transmitter 24. Conduit 23 is provided with a plurality of equally spaced openings 25 shown in Fig. 1 which exhaust into the drum 2G. The size of these openings and their spacing with respect to the tube diameter may be proportioned, in accordance with the prin ciples set forth by Stewart in the aforementioned paper, to impart to the filter the ability to transmit sound waves of frequencies greater than about 300 C. P. S. and to attenuate to a certain extent sound waves of lower frequencies. Connection to the free air from the interior of drum 26 is made by filter device 27, which is similar to filters 16 and 17. This prevents the escape of such higher frequency sound waves as may be radiated from the holes in filter 23.

Vhat is claimed is:

1. A voice mufller for a telephone transmitter comprising a chamber adapted to form a mouthpiece and a breath exhaust passage leading from said chamber to the open air, said passage comprising a tubular ele ment having an expansion chamber coupled thereto, and being proportioned to substantially suppress the transmission therethrough of sound waves corresponding to the higher frequencies of speech.

2. A voice mufller in accordance with claim 1 in which the breath exhaust passage is proportioned to suppress all sound waves above a frequency at least as low as 300 cycles per second.

3. A voice muifler in accordance with claim 1 in which the expansion chamber is connected to the breath exhaust tube at a point intermediate its ends.

4. A voice muffler for a telephone transmitter comprising a chamber adapted to form a mouthpiece and a breath exhaust passage, said passage comprising a tubular element having a side branch passage con nected thereto at a point intermediate it; ends, the component parts of said exhaust passage being proportioned to substantially suppress the transmission therethrough of sound waves corresponding to the higher frequencies of speech, and to relieve the pressure in the mouthpiece chamber due to waves of very low speech frequencies.

5. In combination with a telephone transmitter, a chamber adapted to form a mouthpiece, a breath exhaust passage from said chamber including means adapted to attenuate sound waves corresponding to the higher frequencies of speech, and additional means for relieving excess pressure on the transmitter diaphragm due to the speech waves of low frequencies.

6. In combination with a telephone transmitter, a chamber adapted to form a mouth piece, a breath exhaust passage from said chamber including means for attenuating speech waves of the higher frequencies, and means for compensating the effects of low frequency resonance in said chamber upon the diaphragm of the transmitter.

7. A voice muffler for a telephone transmitter comprising a chamber adapted to form a mouthpiece, a sound wave passage adapted to connect said chamber with a telephone transmitter, and a breath exhaust passage leading from said chamber to the open air, said exhaust passage including an acoustic wave filter adapted to attenuate sound waves above a frequency at least as low as 300 cycles per second, and said first mentioned passage including an acoustic wave filter adapted to attenuate speech waves of low frequencies.

In witness whereof, I hereunto subscribe my name this 18th day of November, A. D. 1925.

WALTER F. SMITH, JR. 

